A principal image can be optically recreated by forming a primary mask containing the principal image and then shining light through the primary mask onto an image receiving surface. Some image features on the primary mask block the light while other image features on the primary mask allow the laser light to be transmitted to the image receiving surface. The transmitted light changes the characteristics of the image receiving surface, recreating the image thereon.
Etch resistant layers known as resists are used widely in the semiconductor industry. Generally, resists may be applied to a wafer and then "exposed" with laser light, so as to impart an image onto the resist. In the image areas where exposure occurs (the "imagewise exposed areas"), the chemical or physical properties of the resist are altered. A chemical process is then applied to the wafer, removing the resist in the imagewise exposed areas (or possibly in the unexposed areas for negative working resists). Removal of the resist uncovers the underlying material, which may now be etched, deposited onto, or made to undergo chemical reactions. For the purposes of this disclosure and the subsequent claims, the word "resist" should not be limited to the traditional use of resists in semiconductor fabrication processes. Rather, "resist" should be interpreted in a broad sense to mean any patterned layer that may be imaged by radiation.
Historically, most resists were "photoresists", which are exposed by the photonic action of light, or "electron beam resists", which are exposed by electron beam radiation. These two types of resists share a common property; they both respond to total exposure, rather than momentary illumination and once the total exposure reaches a threshold level, the resists undergo a chemical or physical change. In optics, "exposure" is defined as the linear integral of illumination over time. For example, a photoresist can be exposed by an illumination of 100 mW/cm.sup.2 for a time of 1 second to yield an exposure of 100 mJ/cm.sup.2, or equivalently, can be exposed by 1000 mW/cm.sup.2 for 0.1 seconds to yield the same total exposure. This phenomenon, known as the "reciprocity law", governs the imaging of photo and electron beam resists, as they are sensitive to the total exposure to radiation.
Ignoring secondary effects, such as the coherence of the light source, photo and electron beam resists behave according to a linear function of power and time; as such, they follow the principle of linear superposition, which implies that f(a+b)=f(a)+f(b). That is, the total exposure response of a photoresist to radiation made up of multiple parts is equal to the sum of the exposure responses to each element of radiation, as if it were applied separately.
Because of the superposition principle and the reciprocity law, photo and electron beam resists require high contrast ratios and low leakage. For example, an imaging system may have stray light leakage of 1% (i.e. when the illumination is "off", the light level does not drop to zero, but only to 1% of the "on" state). In this situation, the total exposure due to stray light may approach the same level as the nominal exposure if the stray light is left "on" for a long period of time. Light leakage may also cause problems for storage and handling of photoresists, because photoresists can be exposed by ambient light.
A more significant disadvantage is caused when trying to image high resolution features onto a photoresist. The point spread function of an optical system causes a "spreading" of light from each image feature. This spreading may produce an overlap between the light which exposes adjacent image features, effectively reducing the system resolution. Obviously a reduction in resolution is an undesirable characteristic, particularly in the semiconductor fabrication industry, where image features are reaching the sub-micron level.
Recently, a new type of resist, known as a thermoresist, has been developed for use in the manufacturing of printing plates and printed circuit boards. Thermoresists change their chemical or physical properties when a certain threshold temperature is reached, rather than a certain accumulated exposure. Thermoresists do not obey the laws of reciprocity or superposition. In contrast to photoresists, which may be exposed by prolonged exposure to ambient light, a thermoresist will not be exposed by prolonged exposure to ambient (i.e. cool) temperature, because it does not obey the reciprocity law. For this reason, thermoresists are more convenient than photoresists, because they do not require expensive handling procedures to prevent accidental exposure.
An additional advantage of thermoresists is that their exposure is a non-linear process (i.e. superposition does not apply), because stray heat dissipates quickly from the surface of the material. This feature of thermoresists can be exploited to improve the system resolution by dividing a principal image into multiple component images and then individually exposing the component images until the entire principal image is recreated. This technique was the subject of the '378 and '078 applications and is discussed below.
The '078 application discusses how improved image resolution can be obtained on an image receiving surface coated with thermoresist by separating the principal image into a plurality of component images. The component images are individually recorded onto the image receiving surface until the entire principal image is recreated. The technique disclosed in the '078 application involves dividing the principal image on the primary mask into an array of cells, wherein the cell size corresponds to the size of the minimum image feature. It is important to understand throughout this disclosure, that the concept of a "cell" is purely for ease of discussion. There is no actual division of the principal image.
A secondary mask is introduced between the laser source and the primary mask. The secondary mask contains an array of optical elements, which causes the illumination of a subset of cells (i.e. a plurality of exposure points) on the primary mask and the imagewise recording of a corresponding plurality of exposure points on the image receiving surface. The optical elements on the secondary mask have a known configuration and orientation, such that the plurality of exposure points illuminated on the primary mask and the corresponding plurality of exposure points imagewise recorded on the image receiving surface are separated from one another by a predetermined interval. The interval between the individual exposure points illuminated on the primary mask and the corresponding individual exposure points imagewise recorded on the image receiving surface is referred to in the '078 application as the "pitch". It should be understood that the term "pitch" is used in association with the exposure points on the primary mask and the image receiving surface, and not the secondary mask. Rather, the secondary mask has a predetermined configuration and orientation of optical elements, which produce the pitch on the primary mask and the image receiving surface.
A conventional optical stepper, can then be used to translate (i.e. "step") the primary and secondary masks with respect to one another so as to record a second component image. Stepping and recording of component images continues until the entire principal image on the primary mask is transferred to the image receiving surface. Obviously, the total number of stepping and recording procedures required to record the entire principal image depends on the pitch.
A one dimensional representation of the process is depicted in FIG. 1. The secondary mask 1, containing a series of evenly spaced apertures 5, is shown in three different locations (1A, 1B, 1C) relative to the primary mask 2, which carries the principal image 6. The primary mask 2 is divided into cells (a, b, c, d, e, f, g, h, I, j, k). One again, it should be understood that the term "cell" is purely for ease of reference and discussion, as there is no actual division of the primary mask 2.
When the laser source (not shown) impinges on the secondary mask 1 in position 1A, a plurality of exposure points (i.e. cells c, f and i) on the primary mask 2 are illuminated. It can be seen that the apertures 5 of the secondary mask 1 are configured, positioned and oriented so as to create a pitch of three on the primary mask. That is, the interval between the illuminated cells (c, f and i) on the primary mask 1 is three. Light from the laser source is imagewise transmitted through the primary mask 2 in a fraction of the cells c and f, but the principal image 6 contained on the primary mask 2 blocks the light completely in cell i. Consequently, the temperature distribution 3A is produced on the image receiving surface. At a certain threshold temperature t, the thermoresist, which coats the image receiving surface, is exposed, undergoing a chemical change. Plot 4A depicts the resultant component image, with an exposure pattern 7A recorded on the image receiving surface. The exposure pattern 7A represents a component image, which is the imagewise exposure of the plurality of exposure points (i.e. cells) on the primary mask 2 that were illuminated when the secondary mask 1 was in position 1A.
An important aspect of the improved resolution characteristics of thermoresists can be seen by noticing that the component image 7A does not show the effects of the stray heat contained in the "tails" of the temperature distribution 3A, which spread laterally outsid e of the cells c and f. Because this stray heat is below the threshold level t and the thermoresist does not obey the reciprocity law, the regions impacted by the stray heat will quickly cool down and act as if they never experienced the heat. The thermal time constant of typical thermoresist layers is in the range of a few microseconds. Once the heat is allowed to dissipate, adjacent image features can be recorded in close proximity to the existing image features without fear of the "overlap" of stray heat. Without overlap of stray heat, a much higher resolution can be obtained than that of photo and electron beam resists, where stray irradiation caused by the spreading of light or electrons can overlap onto adjacent image features (see also the discussion of FIG. 3 below).
After creating the first component image 7A and a short delay (i.e. to allow the stray heat to dissipate from the "tails" of the temperature distribution 3A), secondary mask 1 is stepped to position 1B, such that the apertures are repositioned over primary mask cells b, e, h, and k. The principal image 6 on the primary mask 2 blocks the light in all of the cells except k, and the temperature distribution 3B is created on the image receiving surface. Plot 4B depicts the cumulative resultant exposure pattern, which shows the combination of component image 7A and 7B. Finally, a third component image 7C is created with the secondary mask 1 in position 1C over primary mask cells a, d, g, and j. The resultant heat distribution is depicted in graph 3C and the cumulative resultant component image 7A, 7B, and 7C is depicted in plot 4C.
Plot 4C clearly shows that the principal image 6 carried by primary mask 2 has been recreated on the image receiving surface. It may also be seen from plot 4C that the "tails" of the heat distributions (i.e. 3A, 3B, and 3C) from the various component images (i.e. 7A, 7B, and 7C) do not affect the overall image 4C transferred to the image receiving surface.
FIG. 2 shows a secondary mask 1 according to the '378 invention, with apertures 8. The apertures 8 of the secondary mask 1 are oriented, configured and positioned so as to create a plurality of exposure points with a pitch of three on the primary mask (not shown).
The primary benefit of separating the primary image into component images is an improvement in the available resolution, which is described with reference to FIG. 3. A limiting factor in the resolution of an optical imaging system is the resolution of the optics themselves. In practical imaging systems, an optical system (not shown) is introduced between the primary mask 2 and the image receiving surface (not shown) in order to focus the imagewise illumination of the principal image 6 onto the image receiving surface. In a prior art, single illumination process, the resolution of the optical system had to be extremely fine. In practice, this resolution had to be approximately sufficient to cause the change in light intensity at the image receiving surface from the smallest image feature to exceed 50% of that of the largest image feature.
This "rule of thumb" requirement is depicted by the relationship between graph 10 and threshold 11. Graph 10 represents the system response of a prior art, single illumination process at the image receiving surface. To resolve the smallest individual transparent image feature 2', the corresponding response 10' at the image receiving surface has to exceed the threshold 11. As can be seen from the graph 10, the response 10' at the image receiving surface is insufficient to recreate the image feature 2', which has a width W. If the overall system power density (i.e. laser intensity) was increased, the height of the entire response 10 would increase, and the particular aspect of the response 10' would also increase, so as to faithfully reproduce the width W of the image feature 2'. However, increasing the laser intensity is not always possible, in a single illumination process the smallest individual opaque feature 2", must also be resolved, meaning that the corresponding response 10" can not be caused to rise above the threshold 11. An increase in the overall system power density would cause the aspect 10" of the system response corresponding to opaque feature 2' to rise above the threshold 11, such that the opaque feature 2' would not be resolved. In a single illumination process, any attempt to improve the reproduction of feature 2' will be at the expense of feature 2". For this reason, the threshold 11 was normally selected to be roughly at the midpoint between the exposure level generated by the largest opaque area (i.e. nearly zero) and the exposure level generated by the largest transparent area (shown as 100% in FIG. 3). Thus, the practical rule for single illumination systems was that the resolution of the optics had to be sufficient to cause the change in light intensity at the image receiving surface from the smallest image feature to exceed 50% of that of the largest image feature.
The resolution problem explained above can be completely solved by dividing the principal image into component images. FIG. 3 shows a secondary mask 1 with a pitch of two. Assume that when the secondary mask 1 is in position 1A, that the optical system only has sufficient resolution to produce the response given by plot 10A at the image receiving surface. The response corresponding to feature 2' only has height A (i.e. much less than the required threshold 11). However, the total intensity of the laser source (not shown) can be increased until plot 10A becomes plot 14, crossing the threshold 11 to generate the correct feature size W. The secondary mask 1 can then be moved to position 1B and the plot 10B can be "scaled up" in the same fashion, by increasing the laser intensity.
To exploit the above mentioned benefits of using component images, the interaction between image features 2' and 2" requires that the apertures 5 of the secondary mask 1 are oriented, sized and configures in a particular manner. The size of the exposure points illuminated on the primary mask (i.e. the cells) must be the same size (or smaller) than the smallest image feature and the pitch of the exposure points must be two or greater (i.e. adjacent cells must not be illuminated). If these conditions are met, then adjacent image features on the primary mask 2 are always exposed during different component images. When adjacent image features are imaged during different component images, then the intensity of the laser source can be increased as described above.
In theory, the response level A can be a very small fraction of the threshold level 11. In practice, however, the level A is limited by the interaction between image features. The smaller the response A is, the higher the increase in laser intensity must be in order to reach the threshold 11. Consequently, the exposure points on the primary mask (i.e. cells) must have a greater pitch in order to avoid interaction between the "neighboring" cells of a component image. Because the cell size corresponds to the minimum feature size, a reduction in the response level A corresponds to a lower cell size, a larger pitch and a higher resolution. It is well known that the density of integrated circuits requires the use of the smallest features and highest resolution possible; this corresponds to a small A value, a higher pitch and lower cell size. Consequently, the process of dividing a principal image into multiple component images is well suited to semiconductor processing technology.
FIG. 4 depicts the separation of a principal image 2 into four component images (2A, 2B, 2C and 2D). The insert 15 demonstrates how the secondary mask (not shown) would have had to have been configured and oriented so as to produce component images (2A, 2B, 2C and 2D) with a pitch of two. Component image 2A was exposed when the secondary mask was positioned to illuminate cells in odd rows and columns. Similarly, the secondary mask position for component image 2B would allow illumination of cells in odd rows and even columns, 2C would allow illumination of cells in even rows and odd columns and 2D would allow illumination of cells in even rows and columns.
The '078 application discloses that a secondary mask may be composed of a two dimensional array of lenslets rather than a set of evenly spaced apertures. The disadvantage of apertures is that a significant percentage of the light from the laser source is attenuated by the secondary mask and does not contribute to the exposure at the image receiving surface. On the other hand, a lenslet array can be used to concentrate the light energy. Accordingly, a secondary mask comprising a lenslet array can be used to focus the light energy onto the plurality of exposure points of the primary mask in a manner which increases the imaging efficiency and reduces the exposure time. The lenslets in the secondary mask can be dimensioned and positioned so as to illuminate cells on the primary mask with a certain pitch. This may be done by controlling the magnification, dimensions, spacing, orientation and numerical aperture of the lenslets in the secondary mask as well as the spacing between the secondary and primary masks.
The '078 application also discloses a method and apparatus operative to move the secondary mask on two axes, while maintaining precise control over its position and registration. Accurate control of the relative position of the two masks is important to ensure that the various component images are exposed properly, so that they will recombine to form the entire principal image on the image receiving surface.
The registration and position control of the '078 application is depicted in FIGS. 5, 6 and 7. FIG. 5 shows the apparatus according to the '078 invention, wherein the position control of the secondary mask 1 is accomplished using a system of piezo-electric actuators 21 and 22, photo-detector 25 and linear grating markings 24 located on the edge of the primary mask 2. As depicted in FIG. 6, light 17a travelling through the lenslets 30 of the secondary mask 1, will be focused down to the size of a single cell when it strikes the grating markings 24 on the primary mask 2. The linear grating markings 24 are created with line and spacing widths equivalent to that of the cells in the imaging process. The response at the photo-detector 25 is reproduced in FIG. 7. When the light is focused squarely onto an opaque grating line, then the resulting light intensity (FIG. 7) at the photo-detector 25 will be at a minimum. Conversely, when the light is focused squarely onto a grating space, the resulting light intensity (FIG. 7) will be at a maximum. In this manner, the photo-detector 25 is able to detect whether the secondary mask is properly registered and comparing amplifier 32 is able to control the piezo-electric actuators 21, 22 so as to move the secondary mask. Once a first peak in the detected signal (FIG. 7) is located (i.e. the light is focussed onto a space on the grating 24), then the secondary mask must only be moved a discrete number of peaks, because the peaks of detected signals correspond to the spacing of the grating, which corresponds in turn with the cell size used in the imaging process.
FIG. 8 shows a configuration with three sets of position marking 24a, 24b, and 24c and three corresponding actuators 21a, 21b, and 21c. The two sets of position markings 24a and 24b coupled with two sets of actuators 21a and 21b can be used to control the motion of the secondary mask in one dimension and any potential rotation of the secondary mask. A single set of markings 24c and a single actuator 22 along the transverse axis are used to control the motion of the secondary mask along the transverse axis.
The '078 application also discloses an additional technique that can be used to further concentrate the intensity of the light in the system. The method is depicted in FIG. 5 and involves creating a concentrated line of laser light 17a using mirror 19 and lens 20. Actuator 18 may then be used to "scan" the line 17a across the surface of the secondary mask 1. According to this process, the formation of a single component image would require that the line of light 17a be completely scanned across the width of secondary mask 1. At the expense of slightly more time for each component image, this technique would greatly increase the power density of the light received at the image receiving surface 28. If the laser is a pulsed laser, then the line width and the scan rate of the line 17a are constrained by the fact that at least one pulse must be delivered to each cell of a component image. This technique of using a concentrated scanning line 17a is particularly well suited for optical steppers of the "step and scan" variety, as many of the scanning optics are already provided.
The '078 application discussed the technique of using primary and secondary masks in conjunction with the conventional technology for microlithography known as "optical steppers". In a typical semiconductor fabrication application, the image receiving surface is a wafer and each wafer has several die, which may be cut from the wafer to form chips. Optical stepper devices employ discrete movements (i.e. "stepping") between the various die on the wafer in order to impart an image on to each die. A second type of technology, widely used in the printing industry and recently entering into microlithography involves "scanning". Scanning microlithography devices continuously (rather than discretely) move the wafer with respect to the masks to recreate the image on each die. As such, a method is required to import the technique of using primary and secondary masks described in the '078 application into the presently available scanning semiconductor fabrication devices.
Microlenses were used in prior art to improve the quality of the illumination system in semiconductor equipment and for resolution enhancement. For example, the paper "New Method to Improve the Practical Resolution of Complex Pattern in Sub-Half Micron Lithography" by Xunan et al (SPIE vol. 3334, 1998) discloses improved illumination by using microlenses. This paper, however, does not use the idea of separating the image into subsets, or the use of non-integrating resists which are used in the present invention. Separating the image into subsets is disclosed by U.S. Pat. No. 5,739,898 however it requires a large number of masks in order to separate the image. Also, while the '898 patent uses a non-linear resist it is not free from light integration effects. Non-linear resists are also mentioned in U.S. Pat. No. 5,847,812 and in the paper "Experimental Study on Non-Linear Multiple Exposure Method" by Oki et al (SPIE vol. 3051, pp. 85-93, 1997). These and other non linear resists follow the relationship E=.intg.I.sup.n dt, wherein E=exposure; I=light intensity; n=exponent typically from 1 to 2; and t=time. Resists for which n=2 are also known as "two photon resists" and respond to the square of the illumination. While non-linear resists do not follow the law of reciprocity they are not free from reciprocity and light integration effects, as a prolonged exposure to a low intensity will expose the material; exposure speed will increase faster than the increase in light intensity. This is the basis of U.S. Pat. No. 5,847,232. U.S. Pat. No. 5,851,707 uses image separation but is a linear resist and offers very little improvement.
None of the prior art has the key elements of the present invention, namely using a secondary mask made of microlenses in order to achieve image separation without multiple masks and in particular using this image separation in conjunction with a resist which is completely outside the domain of linear superposition, such as a thermoresist.
Semiconductors are manufactured today using mainly two types of exposure devices, known as steppers and scanners. In a stepper both the mask and wafer are stationary during exposure. In a scanner both the mask and wafer are moving ("scanned") during exposure. The implementation of the invention can be simpler for scanners, as the relative motion between the microlens and mask is provided by the scanning. Since both steppers and scanners are widely used and well known to those skilled in the art, no further details about their construction and operation are given. Both types are manufactured by companies such as ASML (Holland), Canon (Japan) and Nikon (Japan).
It is an object of the present invention to reduce the effects of optical "spreading" and the overlap of adjacent image features and to increase system resolution by providing a method wherein a secondary mask may be used to retain separation between exposure points on the primary mask and the image receiving surface.
It is a second object of the present invention to incorporate the technique of using primary and secondary masks into presently available scanning microlithography devices, so as to recreate a high resolution principal image on an image receiving surface.